Conventional diesel powered systems for engines, factories, and power plants typically produce emissions that contain a variety of pollutants. These pollutants may include, for example, particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, diesel powered engine exhaust emission standards have become increasingly stringent. The amount of pollutants in the exhaust stream may be regulated depending on the type, size, and/or class of engine.
One method that has been considered by engine manufacturers for complying with the regulations of exhaust stream pollutants has included the employment of an oxygen separation device. The oxygen separation device removes oxygen from air being supplied to the cylinders of a conventional engine and then discharges the removed oxygen back into the ambient air. Reducing the amount of oxygen entering the engine cylinders is an effective means for reducing NOx emissions. However, the separation of oxygen from the intake stream requires work which adversely affects fuel efficiency. In addition, by releasing the separated oxygen into the ambient air, the method wastes oxygen that could be used to oxidize additional fuel to increase engine power density.
Another method used to reduce emissions is selective catalytic reduction (SCR). SCR provides a method for removing NOx emissions from internal combustion engine systems. During SCR, a catalyst facilitates a reaction between exhaust-gas ammonia and NOx to produce water vapor and nitrogen gas, thereby removing NOx from the exhaust gas.
The ammonia that is used for the SCR system may be produced during the operation of the NOx-producing system or may be stored for injection when needed. Because of the high reactivity of ammonia, storage of ammonia can be hazardous. Further, on-board production of ammonia can be costly and may require specialized equipment.
One method of on-board ammonia production for an engine is disclosed in U.S. Pat. No. 6,047,542, issued to Kinugasa (hereinafter the '542 patent). The method includes the use of multiple engine cylinder groups for purifying exhaust gas. In the method of the '542 patent, the exhaust gas of two engine cylinder groups may be made rich by controlling the amount of fuel injected into the cylinder groups. Increasing the amount of fuel creates a rich combustion. The rich exhaust gas of the first engine cylinder group may then be passed through an ammonia-synthesizing catalytic chamber to convert a portion of the NOx in the exhaust gas into ammonia. The exhaust gas and ammonia of the first engine cylinder group are then combined with the exhaust gas of a second engine cylinder group and passed through an SCR catalytic chamber where the ammonia reacts with NOx to produce nitrogen gas and water vapor.
While the method of the '542 patent may reduce NOx from an exhaust stream through use of on-board ammonia production, the method of the '542 patent has several drawbacks. For example, varying the amount of fuel in order to control the leanness or richness of exhaust gas might use more fuel than preferred. Furthermore, by using the method of the '542 patent, it may be more difficult to provide adequate and controlled air intake to both cylinder groups.
The present disclosure is directed at overcoming one or more of the problems or disadvantages in the prior art.